Written by cuweathernerd    Wednesday, 25 January 2012 00:00   
Chasing 101: Section 1:7 Forecasting: Skew-Ts Part 1

This is part of the continuing series called chasing 101, a course to help people who are new to chasing learn the fundamental skills to chase productively and safely. They are meant as both information and as a forum for discussion.


This is a continuation of the previous post on CAPE, and assumes you have read and comprehend it.

Make sure you look at the images in this lesson, as they contain the bulk of the information.


CAPE is a nice tool -- but unless you understand why a specific sounding is making the number, it doesn't tell you much in terms of making a good forecast.

The soundings we use are Skew-Ts, and they are serious instruments of science. They are most of the field of thermodynamics for water put into a single diagram -- and so they are (necessarily) complex.

It is good to know what each line on a Skew T represents, because you will end up using all of them at some point.

Because Skew-Ts have a lot of lines, I made this animation to help understand what each line is. CLICK HERE FOR HIGHER QUALITY ANIMATION

The sounding we are using to demonstrate is a normal day from the high plains of Colorado. It isn't measured data, but instead, an output of a model (the NAM). The easiest way to get one of these model soundings is to open www.twisterdata.com and click anywhere on the map. That'll bring up the sounding for that model at that point in space and time.


All these lines are really the result of lots and lots of math -- and they let you do complex calculations without knowing the math.

One of the easiest calculations -- and the most useful -- is finding the lifted condensation level (the LCL), which is the height you must lift a parcel from the surface before it saturates. This is a good measure of the height of the base of clouds.

I made an animation showing the process of finding the LCL: CLICK HERE FOR HIGHER QUALITY ANIMATION

 

There are more complex ways of calculating the LCL -- they involve averaging the temperature in the lowest part of the atmosphere or other techniques. Computers are great at this -- and its ok to let them do this. So if you see a parameter like mixed layer LCL, this is where that number is coming from.


All of that is well and good, but this doesn't tell us a ton about severe weather. I mean, most people say the sounding is the heart of their forecast. A big reason for this is that CAPE is actually computed using a Skew-T.

The process involves making a parcel trace for your surface parcel -- effectively, figuring out what will happen if you started to lift the air from the surface higher in to the air.

The start is just like finding an LCL, and only adds one step from there.

This is getting to the heart of why we do sounding analysis -- we see that we have a lot of CAPE, but a little cap blocking it off. If we could somehow lift our parcel past that cap, we'd get a big thunderstorm because we'd have free access to the CAPE. Because there is this potential for the CAPE to fire if we could overcome a little CIN, we call this a loaded gun sounding.

Not only that, but it tells us about the quality of our CAPE -- here, we have a really big temperature difference between our parcel and the environment -- you can see that in how "fat" the cape is. Not only that, but you can see it is quite dry about 800mb, and moist air rises faster in dry environments (moist air is less dense than dry). In short, this is some high quality CAPE.


So let's pretend that our sounding changed a bit later in the day. Then, our thunderstorms would really get going. And our sounding helps us to understand how that thunderstorm might look. Because we know the LCL is a cloud base and that our parcel will rise at least through all the CAPE (assuming the CIN is overcome), we can also guess a bit as to how high our cells will get.

You can think of a sounding of working like this: CLICK HERE FOR HIGHER QUALITY ANIMATION

So we might guess from these soundings that tall thunderstorms -- maybe 45k feet -- are possible.

Indeed, the CAPE this day was concerning, leading the SPC to issue PDS watches and a high risk for tornadoes. It ended up producing a major tornado outbreak.


Skew-Ts are immensely powerful tools, and we've just covered a teeny bit of what you can do with them. We'll cover a few more tools in the next lesson to round out a basic chasing tool set on instability. We'll move from there towards what fires our storms -- fronts, shortwaves, longwaves, and jet streaks.


 

The above is an article written by Reddit user cuweathernerd and has been modified by Ryan Lehms. The original article and discussion can be found here:

http://www.reddit.com/r/stormchasing/comments/owxlg/chasing_101_section_17_forecasting_skewts_part_1/

Last Updated ( Sunday, 11 August 2013 11:26 )
 

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